This invention relates to apparatus and methods for providing compensation of polarization-dependent distortion such as Polarization Mode Dispersion (PMD), in high-speed optical transmission networks and systems.
Fibre-optic transmission systems are now being developed for tens of gigabits-per-second (Gbit/s) communication channels, whilst large volumes of 10 Gbit/s systems are being fully deployed into existing networks. Various potential limits are approached as the performance of such transmission systems in pushed further. The phenomenon of polarization mode dispersion, PMD, is a problem recently attracting a great deal of attention from the telecommunications industry. PMD is a type of distortion that varies from fibre to fibre and is typically of greater magnitude in older fibres. PMD is also a random phenomenon, varying with both time and optical frequency. Whilst service providers are reluctant to invest in new fibre routes, PMD may restrict the deployment of new systems aver the older fibre routes of their network. In a small number of fibres, PMD will give rise to distortions so large that a 10 Gbit/s optical transmission system cannot be reliably deployed over the route. The impact of PMD scales linearly with system bit-rate, hence PMD will become a greater problem ax the bit-rates of systems are increased. It is for these reasons that PMD solutions have to be found.
PMD in a fundamental characteristic of both optical fibres and optical components. It arises from the consideration that single mode fibre can actually support two weakly guided modes that are orthogonally polarised. In other words, given an ideal fibre, a pulse can be launched into either of these two polarization modes and propagate through the fibre in that polarization mode alone. A fiber exhibits slightly different refractive indices along different axes, a physical characteristic known as birefringence. Birefringence arises from a variety of intrinsic and extrinsic features of the fibre manufacture. These features include geometric stress caused by a noncircular core, and stress birefringence caused by unsymmetrical stress of the core. Other sources of birefringence include external manipulation of the fibre. External forces will include squeezing the fibre, bending the fibre and twisting of the fibre
In a birefringent fibre, the propagation speed will vary with the launch polarization state into the polarization nodes of the fibre. Consequently, when proportions of the pulse are launched into both polarization axes they travel at different speeds and hence arrive at different times. The magnitude of the difference in arrival times between the fastest and slowest paths (along the two Principal States of Polarisation-PSPs) through the fibre is known as the differential group delay (DGD).
The receiver of a direct detection optical transmission system does not distinguish between the different polarization modes, but simply detects the combination of the different polarization modes. The difference in arrival times of the pulse through the two polarization modes will degrade the quality of the received data.
In a long length of fibre the birefringence is expected to be weak but vary randomly along its entire length. This leads to random mode coupling along the fibre, a process by which the pulse will continuously couple power between the two polarization modes of the fibre. The phenomenon of PMD relates to the random variation of the DGD of the fibre. The DGD is expected to vary randomly over time due to random variations of the fibre birefringence as a result of environmental effects, such as temperature. A consequence of this random variation means that the instantaneous DGD of a fibre cannot be predicted. Instead the DGD of a fibre must be described statistically. The fibre DGD also varies over frequency/wavelength.
The DGD is the first-order consideration of PMD. It makes the assumption that the PMD characteristics of a fibre are constant over the bandwidth of the transmitted data signal. Higher-orders of PMD are considered when the PMD characteristics can no longer be considered constant over the bandwidth of a signal. Higher-order PMD relates to the variation of the PMD characteristics of a fibre with frequency.
In order to compensate for first order PMD, it has been proposed to use a delay line which provides differential delay for different polarization states, in order to reverse the system fiber DGD. A particular class of fibres, known as high birefringence (Hi-Bi) fibres, has been engineered deliberately to have very high, uniform birefringence for this purpose. The fibres have two clearly definable axes with different refractive indices. The propagation speed of a pulse will differ greatly between each axis.
In all-optical PMD compensation, the restoration of PMD distortion is done optically without any optical-electrical conversion. Normally, all-optical PMD compensators consist of a polarization controller and a fixed birefringent delay element, such as a piece of high birefringence optical fiber. The basic concept is to align the principal states of polarization (PSP) of the fiber with the principal axes of the birefringent delay element to reverse the DGD of the system fiber.
This type of compensator requires tracking of variations in the state of polarization of the incident signal, for example using a polarization tracking system. The need for endless control in polarization tracking devices (whether for use in PMD compensators or in other devices) has been recognised. However, endless control systems are generally complicated and expensive to implement.
According to a first aspect of the invention, there is provided a PMD compensator comprising first, second and third compensation elements in series, each for providing variable differential group delay, a first polarization converter being provided between the first and second compensation elements and a second polarization converter being provided between the second and third compensation elements, the polarization converters being selected such that each compensation element applies a variable differential group delay to a different orthogonal axis.
This device provides PMD compensation by providing DGD compensation independently along three different orthogonal polarization axes. This enables PMD to be cancelled without the need for an endless polarization controller.
One polarization converter preferably comprises a 45 degree polarization rotator and the other polarization converter comprises a quarter wave plate and a 45 degree polarization converter in cascade. In this way, one converter converts between vertical linear and 45-degree linear polarization and the other converts between linear and circular polarization. This enables three orthogonal axes of the Poincare sphere to be compensated by the three compensation elements.
In one specific example, the first polarization converter comprises a xe2x88x9245 degree polarization rotator and the second polarization converter comprises a quarter wave plate and a +45 degree polarization converter in cascade.
Each compensation element must provide tuneable variable DGD, and various possible designs of compensation element are available.
The control of each compensation element may be in response to a single control parameter, for example the magnitude of the differential group delay at the output of the device. As each compensator is minimising the DGD of one of three orthogonal DGD components present in the input, a single control parameter can be used. However, a dither signal may be applied to the control signal for each compensation element, so that the effect of tuning each compensation element can be detected in the output. Thus, a corresponding spectral component of the differential group delay at the output of the device is used to from the basis of control of each compensation element.
According to a second aspect of the invention, there is provided a method of providing PMD compensation, comprising;
introducing a first differential group delay to minimise a first component of the differential group delay of an input signal;
introducing a second differential group delay to minimise a second component of the differential group delay, orthogonal to the first component, of the input signal; and
introducing a third differential group delay to minimise a third component of the differential group delay, orthogonal Lo the first and second components, of the input signal.
This method provides independent minimization of the three orthogonal components of DGD in an input signal, which can enable the DGD to be cancelled more effectively.
In order to enable each compensation stage to operate on a different axis of the DGD, a polarization rotation is preferably performed between the first and second or second and third differential group delay sections, and a linear to circular polarization conversion is performed between the second and third or first and second differential group delay sections.
The levels of the first, second and third differential group delays are preferably selected such as to minimise a control parameter, for example the magnitude of the differential group delay after the introduction of the third differential group delay.
According to a third aspect of the invention, there is provided a device for compensation of polarization-dependent distortion, comprising first, second and third compensation elements in series, each for providing variable polarization-dependent distortion compensation, a first polarization converter being provided between the first and second compensation elements and a second polarization converter being provided between the second and third compensation elements, the polarization converters being selected such that each compensation element applies variable compensation to a different orthogonal axis.
The polarization-dependent distortion may comprise polarization dependent loss, and the compensation elements may then comprise variable polarization dependent loss elements.